The present invention relates generally to footwear. More particularly, the present invention relates to methods and structures for removing heat, odors, and/or moisture from an article of footwear such as a shoe or a boot.
As is well known, feet generate heat like other body parts. When feet are enclosed in shoes, for example, the shoes tend to retain the heat, which causes the feet to sweat. Sweat or perspiration is a mechanism for the body to flush wastes, regulate body temperature, and help keep the skin clean and pliant. Sweating can be a response to hard working muscles, a hot environment, or over stimulated nerves. It is the evaporation of perspiration from the skin that is the means of heat transfer. Because evaporative heat loss is the major mechanism of cooling, the body is constantly sweating as heat is transferred away from the core to the skin's surface. This is also referred to as insensible water loss, and is so named because it usually goes unnoticed. Insensible water loss, occurring via respiration and the skin (trans-epithelial) cannot be prevented, is a major source of heat loss for the body, and is not controlled by the body's regulatory system.
Eccrine sweat glands are the most numerous type of sweat glands and are found all over the body. They are particularly concentrated in the palms of the hands, soles of the feet and the forehead. There are approximately 250,000 sweat glands in a pair of feet. This represents more sweat glands per square inch of skin than any other part of the body. On average, sweat glands in the feet excrete as much as a half-pint of moisture a day. Moisture (also referred to herein as moisture vapor) includes a liquid that is diffused or condensed in relatively small quantity and dispersed through a gas as invisible vapor. Although sweat's main function is to control body temperature by evaporation, it also serves to keep the skin moist. As such, the skin covering the foot can withstand the constant flexing and friction that happens with locomotion, preventing the skin from rapidly becoming dry, irritated, and uncomfortable.
In the footwear industry the ‘in shoe’ experience of the foot is often referred to as the microclimate of the foot-so called due to the environment that is created when a foot is covered, even partially, by footwear. This zone includes the air trapped by footwear underneath and around the foot. When the footwear substantially covers the foot or just covers the forefoot, and especially if the entire foot is covered in a traditional shoe, boot or like type of footwear, the foot microclimate is a factor in both foot comfort and foot health. The feet experience the highs and lows of temperature and humidity with greater variation than elsewhere on the surface of the body. In part, this is because the feet are further from the heart than any other part of the body. Consequently, by the time the blood arrives to the feet, there has already been considerable cooling.
The core temperature of the body varies only slightly with large changes in environmental temperature, but there is a much greater effect on the feet. For example, with an air temperature of sixty-eight degrees, the surface temp of the foot at rest, without covering (protection) is approximately seventy-two degrees while the core body temperature remains constant at approximately ninety-eight degrees. In this instance, it is necessary to provide thermal insulation to the foot in order to maintain a comfortable foot temperature. Providing a waterproof barrier can also be extremely important feature of footwear in a wet environment as cold water temperatures and the resulting evaporative heat loss can leave the feet very cold and uncomfortable. In extreme situations, this can lead to frostbite and other deleterious conditions.
In the opposite environment, requirements for protective and other footwear are quite different. During activity in hot environments, foot temperature becomes elevated and the humidity within the shoe increases sharply. Of course, materials used in the upper and in the sock will affect both the in-shoe temperature and humidity. Materials that form a barrier to air permeability and water vapor transmission quickly create moist, hot, uncomfortable environments as evaporation of sweat is severely limited, as is air movement within the shoe microclimate. This can also result in an undesirable, malodorous condition.
In warm environments, most protective footwear creates an uncomfortable micro environment, and only the most breathable footwear can provide reasonable comfort for the wearer. A major factor that influences the rate of evaporation is the relative humidity of the air around the foot. If the air is humid, then it already has water vapor in it, probably near saturation, and cannot take any more. Therefore, sweat does not evaporate and cool the body as efficiently as when the air is dry.
Moist, hot air commonly creates a very uncomfortable experience for the foot. This is generally regarded as a compromise in footwear between protection and comfort. An ideal shoe would provide all the needed protection without reducing air flow around the foot. More ideally, air flow should be limited when the foot is cool and increased as foot temperature increases.
Two dynamic factors heavily influence footwear comfort, namely the external environment and the internal environment or microclimate. Protection from the external environment is important for several reasons, including protection from ground level objects or surfaces, moving objects and external climate (temperature and weather conditions). Protection may be provided through design and/or use of materials having characteristics such as insulation, durability, waterproofing and breathability.
Typically, the type of footwear (sandal, work boot, hiker, casual, golf shoe, running shoe, sneaker, etc.) is chosen based on intended use and climate considerations. For example, a runner would likely choose an insulated, waterproof running shoe during cold, wet weather and a light, breathable shoe for use in warm weather. A hiking shoe is commonly chosen for protection from objects in the hiking environment and external climate. Regulation of the footwear's internal environment is extremely important in order to provide a consistently comfortable micro environment as external and internal conditions change during activity. The internal environment is heavily influenced by heat and sweat produced by the body, which, as discussed above, is a method of controlling body temperature.
It is well known that a critical problem with protective and/or enclosed footwear is that moisture vapor from perspiration is trapped and cannot escape the footwear. In order to provide increased comfort, wicking characteristics have been applied to sock and liner materials. At least some of the moisture is absorbed within the footwear, the sock and/or the liner materials. Once such materials become saturated they can loose their effectiveness and create an unpleasant environment for the foot.
Such materials can quickly reach a saturation point as the moisture within the shoe typically does not have an efficient method of evaporation. Skin also absorbs moisture when in a saturated environment. Skin is softened by the absorbed moisture, becomes more sensitive to pressure, and also becomes prone to abrasion and fungal infection. Thus, the reduction in airflow around the foot and within an article of footwear presents a significant number of challenges to creating a comfortable environment for the foot.
Another consideration is that although there have been advances in materials that claim a level of breathability or airflow through (stand alone) material, testing has proven that when such materials are combined with traditional constructions of protective footwear, the breathability is greatly reduced due to construction types and the necessity of the materials to be used in combination with additional ‘non-breathable’ materials and adhesives during the construction process. Therefore, a need exists for advanced footwear and foot covering materials and methods of manufacture that enhance air flow and convective cooling of the foot while providing comfort, support and other common features of footwear.